1. Field of the Invention
The present invention relates to a variable wavelength light source for varying a difference in frequency between two coherent light sources.
2. Prior Art
FIG. 6 is a schematic diagram of a variable wavelength light source. In FIG. 6, denoted at 1 is a light source which emits coherent light, 2 is a variable wavelength light source, 3 is an optical branching device, 4 is an optical branching device, 7 is an optical coupler, 8 is a light detector, 9 is a signal generator, 10 is a phase comparator and 11 is a low-pass filter (referred to as an LPF hereinafter).
In FIG. 6, the light source 1 emits coherent light having a frequency of f.sub.0 and the variable wavelength light source 2 emits light having a frequency of f.sub.a. The optical branching device 3 branches light emitted from the light source 1 to the outside as reference light and to the first input terminal of the optical coupler 7. The optical branching device 4 branches light emitted from the variable wavelength light source 2 to the outside as outputted light and to the second input terminal of the optical coupler 7. The optical coupler 7 combines light having a frequency of f.sub.0 from the optical branching device 3 with that having a frequency of f.sub.a from the optical branching device 4.
The light detector 8 detects the combined light supplied thereto from the optical coupler 7 and outputs a signal having a frequency of .DELTA.f.sub.1 which is a frequency difference between light emitted from the light source 1 and that from the variable wavelength light source 2. The signal generator 9 supplies a signal having a frequency f.sub.SG2 to the phase comparator 10. The phase comparator 10 compares in phase the signal having a frequency of .DELTA.f.sub.1 supplied thereto from the light detector 8 with a signal having a frequency Of f.sub.SG2 outputted from the signal generator 9 to convert the phase difference therebetween into voltage and output the same. The LPF 11 removes the higher frequency component from the signal outputted from the phase comparator 10 and outputs the remaining light to the variable wavelength light source 2. Upon this, the variable wavelength light source 2 is controlled to make the output of the phase comparator 10 approach zero. As a result, the oscillation frequency of the variable wavelength light source 2 is locked on a frequency which is a sum of the oscillation frequency of the light source 1 and that of the signal generator 9.
The operation of a conventional variable wavelength light source illustrated in FIG. 6 will be described hereinafter with reference to FIGS. 7 and 8. In FIG. 7, denoted at f.sub.0 is the oscillation frequency of the light source 1, f.sub.a is that of the variable wavelength light source 2 before control and .DELTA.f.sub.1 is a frequency difference between light emitted from the light source 1 and that emitted from the variable wavelength light source 2. In FIG. 8, denoted at f.sub.b is the oscillation frequency of the variable wavelength light source 2 in case the same is under control. Denoted at f.sub.SG2 is the frequency of outputted signal of the signal generator 9.
The frequency difference between the light source 1 and the variable wavelength light source 2 is obtained from the formula f.sub.b -f.sub.0 =f.sub.SG2. Difference signal .DELTA.f.sub.1 becomes .DELTA.f.sub.1 becomes to be equal to f.sub.SG2 under control, so that the frequency difference .DELTA.f.sub.1 between the light source 1 and the variable wavelength light source 2 can be varied by operating the signal generator 9 to vary f.sub.SG2.
The conventional variable wavelength light source having the construction illustrated in FIG. 6 has a problem that the actual variable range of frequency difference is limited to the order of several GHz to several tens GHz since the variable range of frequency difference between the light source 1 and the variable wavelength light source 2 is equal to that of the signal generator 9.